1. Field of the Invention
The present invention relates to an electronic apparatus such as a camera using a battery, and in particular, to a flash device for the camera.
2. Description of Related Art
Recently, lithium batteries are commonly used as power supplies for cameras. The lithium batteries have a feature of small self-discharge and, therefore, are adequate for electronic apparatuses that are intensively used in some occasions and are not used for some extent of period until next occasions.
In addition, in an electronic apparatus such as a camera not using silver-halide film, for example, a VTR or a digital camera that uses a CCD or the like that is an imaging device, a secondary battery such as a nickel-cadmium battery (Ni--Cd battery) that is rechargeable is commonly used. The Ni--Cd battery has problems of memory effect, which makes charged electric charges disable to be taken out if the Ni--Cd battery is left in the charged state for long time, and of large self-discharge. Therefore, although the Ni--Cd battery is adaptive for a VTR, a digital camera, and the like that use the charged electric charges in a short time, the Ni--Cd battery is not commonly used as a main power supply for a silver-halide camera or the like, but it is presently used as an auxiliary power supply for a flash device and a motor drive device.
By the way, recently, electronic apparatuses adopting lithium ion batteries that are secondary batteries similarly to Ni--Cd batteries have increased. Since this lithium ion battery has a characteristic of self-discharge being smaller than that of the Ni--Cd battery, and has no memory effect, the lithium ion battery can be stored in the charged state for a long time and is convenient also as a backup battery. Hence, the lithium ion battery is mounted also as a power supply for a portable telephone, a VTR, a digital camera or the like.
Although a lithium ion battery has the above-described merits against a Ni--Cd battery, as demerits, it has problems that it is weak to overcharge and becomes not functional as a battery due to overdischarge. Therefore, several safety circuits become necessary as measures against these problems in an apparatus having large current consumption, such as a flash device.
For example, a fully charged voltage of a lithium ion battery is about 4 V per cell, and a tripping voltage of an overdischarge protection circuit is about 2.5 V. If the lithium ion battery is used for a common flash device, the voltage of the battery lowers to a voltage equal to or lower than the tripping voltage of the overdischarge protection circuit due to large current consumption of the flash device, and hence an inconvenience that the output of the battery is shut off arises.
In addition, also in the case of continuously using a large current, an inconvenience that the current is limited due to an increase of resistance of a PTC device that is called a polyswitch arises. Therefore, a circuit which limits a current supplied to a booster circuit of the flash device, and the like, become necessary.
Furthermore, it is desirable in a portable apparatus to be able to use a plurality of power supplies. For example, there have been commercially produced cameras in which AA-sized alkaline manganese batteries and lithium batteries, which are primary batteries, can be selectively used.
Moreover, a camera including a flash device in which a lithium battery, which is a primary battery, and a lithium ion battery or the like, which is a rechargeable, secondary battery, can be selectively used has a problem that charging performance of the primary battery is made lower by the current limiting circuit necessary for use of the secondary battery.